This invention relates to drum brakes, in general, but in particular to drum brakes for vehicles. It is an object of the invention to solve the problem of prevention of car skidding caused by excessive brake torques at the rear wheels in relation to the brake torques at the front wheels. A further object of the invention is to maintain an adequate and effective brake torque even with unfavorable conditions of brake shoe friction.
In order to insure that braking torques on front and rear wheels are properly proportioned to avoid unsymmetrical wheel locking and car skidding, means, such as a proportioning valve, are used to distribute the hydraulic brake actuating force at the front and rear wheels so as to maintain a fixed ratio of rear to front brake torques. However, such means are made less effective if the coefficient of sliding friction between the brake shoes and the brake drum varies in a random and unpredictable manner due to such factors as corrosion, wear, abrasive dust, moisture, and other chemical and physical changes. An additional important factor that works against the desired brake torque proportioning is the presence of the leading brake shoe with positive feedback in the existing conventional drum brakes. This positive force feedback magnifies the brake torque changes due to changes in the sliding coefficient of friction. This is especially serious when a comparatively small increase in the sliding coefficient results in a comparatively large increase in the brake torque in a steeply non-linear exponential or hyperbolic manner.